1. Field of the Invention
The present invention relates to a method of processing seismic data.
2. Description of Related Art
Seismic data is collected in order to analyse the sub-surface of the earth, in particular for hydrocarbon exploration. Seismic data for analysing sub-surface structures may be collected on land or, over water, using sea-going vessels. In order to obtain the data, a seismic source which may comprise explosives (on land) or an impulse of compressed air or airguns (at sea) is provided. The seismic data signals reflected by the various layers beneath the surface of the earth are known as traces and are sensed by a large number, typically hundreds, of sensors such as geophones on land and hydrophones at sea. The reflected signals are recorded and the results are analysed to derive an indication of the layer formations beneath the sub-surface. Such indications may then be used to assess the likelihood of hydrocarbon deposits.
The analysis of the results to derive an indication of layer formations, however, is not straightforward. Particularly where the materials of the sub-surface of the earth vary laterally, there may be more than one signal path between the seismic source and a point within the sub-surface which reflects the signal. Typically, the same will be true of the return path between the reflecting point and a respective seismic sensor, such as a geophone or a hydrophone. If a case of three different paths in each direction is considered, there will be nine different round-trip routes by which a signal can travel from the seismic source to the seismic sensor from a single reflection point. Cost-effective analysis using all of these possible paths is impossible, so some means of simplifying the processing is required.
In “Green's Functions for 3D Pre-stack Depth Migration” published in the EAGE 57 Conference and Technical Exhibition in Glasgow, Scotland on 29 May to 2 Jun. 1995, the following two prior art techniques for reducing this complexity are discussed. It will be understood that the signals resulting from a single reflection point will generally arrive at the geophone at different times and with different amplitudes dependent upon the distance of the path travelled and the sound-propagating characteristics of the subsurface layers through which the sound waves have passed. Consequently, there are a number of “ray paths” through the sub-surface of the earth which relate to a signal reflected by a single reflection point. One proposed solution to the complexity of the numerous ray paths is to select a so-called first arrival signal. This will be the arrival signal (or “arrival”) corresponding with the fastest propagating seismic signal. However, one drawback of this technique is that the first arrival is rarely the strongest signal and often contains too little energy to provide reliable and accurate analysis. However, the methods for calculating the first-arriving travel time tend to be cheaper and simpler than other methods.
Some of these methods are commonly (and confusingly) referred to as “shortest path” methods, although they actually compute a shortest travel-time path, rather than a shortest physical ray length path. Articles describing this type of method may also be found in Geophysics, Volume 56, No. 1, January 1991, T. J. Moser, “Shortest path calculation of seismic rays”, pages 59–67; Geophysics, Volume 58, No. 7, July 1993, Robert Fischer et al., “Shortest path ray tracing with sparse graphs”, pages 987–996; and Geophysics, Volume 59, No. 7, July 1994, T. J. Moser, “Migration using the shortest-path method”, pages 1110–1120. References to the process of computing the shortest travel-time raypath (rather than the shortest ray length raypath), can be found in these articles on page 59, abstract, line 4; page 987, column 2, line 20; and page 1111, column 2, line 37, respectively.
Another prior art technique is to select the arrival having the maximum amplitude. However, selection of this arrival is not necessarily straightforward because the model of the sub-surface will generally only be approximate. The maximum amplitude arrival will only provide the best single arrival as long as the estimates of amplitude are correct. Another difficulty with the maximum amplitude arrival is that the choice of arrival can switch rapidly back and forth between branches. However, improvements using the maximum-amplitude arrival over use of a first arrival have been observed in the prior art reference first identified above.
It is an object of the present invention to provide a method of processing seismic data which ameliorates the disadvantages of these prior art techniques.